252 MANUAL OF THE MOLLUSCA. 



the inner layer of shell, or from specimens in which this layer is replaced by 

 spar. 



The necessities of geologists have compelled them to pay very minute 

 attention to the markings in the interior of shells, to their microscopic texture, 

 and every other available source of comparison and distinction. It must not, 

 however, be expected that the entire structure and affinities of molluscous 

 animals can be predicated from the examination of an internal mould or a 

 morsel of shell, any more than that the form and habits of an extinct quad- 

 ruped can be inferred from a solitary tooth or the fragment of a bone.* 



The systematic arrangement of the bivalves now employed is essentially 

 that of Lamarck, modified, however, by many recent observations. The 

 families follow each other according to relationship, and not according to 

 absolute rank; the Venerida are the highest organized, and from this culmi- 

 nating point the stream of affinities takes two courses, one towards the Myas, 

 the other in the direction of the oysters ; groups analogically related to the 

 Tunicaries and Brachiopoda. 



SECTION A. ASIPHONIDA. 

 a. Pallial line simple : Integro-pallialia. 



Fam. 1. Ostreidse. 



2. Aviculidae. 



3. Mytilidae. 



4. Arcadse. 



5. Trigoniadse. 



6. UnionidEe. 



SECTION B. SIPHONIDA. 



7. Chamidee. 



8. Hippnritidse. 



9. Tridacnidse. 



11. Lucinidse. 



12. Cycladida?. 



13. Cyprinidse. 



10. Cardiadse. 



b. Pallial line sinuated: Sinu-pallialia. 



14. Veneridse. 



15. Mactridse. 



16. TellinidEe. 



17. Solenida?. 



18. Myacidse. 



19. Anatmidse. 



20. Gastrochaenidae. 



21. Pholadidse. 



The characters which have been most relied on for distinguishing these 

 groups and the genera of bivalves are the following, stated nearly in the 

 order of their value: 



1. Extent to which the mantle-lobes are united. 



2. Number and position of muscular impressions. 



3. Presence or absence of zpallial sinus. 



4. Form of the foot. 



5. Structure of the branckiee. 



* Etudes Critiques sur les Mollusques Fossiles, par L. Agassiz, Neuchatel, 1840. 



